The present invention relates to a DC solenoid suitable for use with a solenoid valve and the like requiring water- and explosion-proofness.
A type of DC solenoid disclosed in JP-A(U)-59-107108 is such that as shown in FIG. 1 a thermistor 2 having a positive temperature coefficient is connected in series to a coil 1 for attracting a plunger and is also connected to a DC power supply through a main switch 3.
This thermistor 2 has such a characteristic that the resistance value thereof increases sharply with temperature in a range higher than a predetermined level. For this reason, although the resistance value of the thermistor 2 is small with a large current flowing in the coil 1 at the time of starting the operation, the temperature of the thermistor 2 increases by self-heating and ambient temperature with the lapse of time and the resistance value thereof increases greatly, thus sharply reducing the current flowing in the coil 1.
If the diameter and the winding specification of the copper wire used for the coil or the characteristic of the thermistor 2 is appropriately set, a large magnetic force is generated at the time of starting the operation and the heat generation of the coil is dampened after starting the operation, so that the coil is prevented from burning while at the same time securing a predetermined holding power.
In applications of this type of DC solenoid to a solenoid valve or the like requiring water- and explosion-proofness, however, the thermistor 2 which is encapsulated integrally with the coil 1 by synthetic resin is so fragile and liable to develop a cracking that it is impossible to apply a high pressure thereto. Thermoplastic synthetic resins such as nylon, polybutylene terephthalate or polyphenylene sulfide cannot be used for encapsulation by injection molding. The injection molding is usually effected under pressure 30-50 Kg/cm.sup.2. As a result, in conventional methods, the coil 1 and the thermistor 2 are integrally sealed by the cast molding using a thermosetting resin such as epoxy resin or polyester resin. The cast molding is usually effected without any pressure.
The cast molding by epoxy resin, however, requires a long time for setting the resin and therefore is low in productivity.
Further, the cast molding, which requires a multiplicity of dies, is high in cost, and since it is difficult to secure the accuracy of the resin-molded parts, cannot be used for integrally molding other component parts such as a connector.